Loans and Allowance - Credit Quality of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 7,088,071	$ 6,751,199
Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	1,655,569	1,493,493
Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	88,504	121,757
Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	668,608	612,219
Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	2,699,629	2,562,691
Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	965,893	962,765
Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	517,303	514,021
Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment [Line Items]
Loans	98,709	86,935
Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment [Line Items]
Loans	1,830	2,527
Other commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	392,026	394,791
Commercial Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	5,431,154	5,099,464
Commercial Pass | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	1,587,560	1,418,401
Commercial Pass | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	68,556	73,800
Commercial Pass | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	629,244	587,906
Commercial Pass | Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	2,551,541	2,408,329
Commercial Pass | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	175,315	185,725
Commercial Pass | Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	25,435	28,554
Commercial Pass | Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment [Line Items]
Loans	1,830	2,527
Commercial Pass | Other commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	391,673	394,222
Commercial Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	100,674	154,573
Commercial Special Mention | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	18,085	51,336
Commercial Special Mention | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	7,144	27,502
Commercial Special Mention | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	2,533	828
Commercial Special Mention | Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	67,043	70,074
Commercial Special Mention | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	5,119	4,376
Commercial Special Mention | Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	750	457
Commercial Special Mention | Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment [Line Items]
Loans
Commercial Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	154,739	129,218
Commercial Substandard | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	49,924	23,386
Commercial Substandard | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans	12,804	20,018
Commercial Substandard | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	10,200	981
Commercial Substandard | Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	78,694	79,769
Commercial Substandard | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	2,377	4,209
Commercial Substandard | Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	387	286
Commercial Substandard | Lease financing receivables, net of unearned income
Financing Receivable, Recorded Investment [Line Items]
Loans
Commercial Substandard | Other commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	353	569
Commercial Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans		2,407
Commercial Doubtful | Commercial and industrial loans
Financing Receivable, Recorded Investment [Line Items]
Loans		370
Commercial Doubtful | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans		387
Commercial Doubtful | Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans		1,536
Commercial Doubtful | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans		114
Commercial Doubtful | Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans
Commercial Loss
Financing Receivable, Recorded Investment [Line Items]
Loans		50
Commercial Loss | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans		50
Commercial Loss | Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans
Consumer Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	1,395,161	1,354,790
Consumer Performing | Agricultural production financing and other loans to farmers
Financing Receivable, Recorded Investment [Line Items]
Loans
Consumer Performing | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	26,494	22,374
Consumer Performing | Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	2,350	2,980
Consumer Performing | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	778,086	759,900
Consumer Performing | Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	489,631	482,661
Consumer Performing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment [Line Items]
Loans	98,600	86,875
Consumer Non-Performing
Financing Receivable, Recorded Investment [Line Items]
Loans	6,343	10,697
Consumer Non-Performing | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans	137	130
Consumer Non-Performing | Commercial and farmland
Financing Receivable, Recorded Investment [Line Items]
Loans	1	3
Consumer Non-Performing | Residential
Financing Receivable, Recorded Investment [Line Items]
Loans	4,996	8,441
Consumer Non-Performing | Home equity
Financing Receivable, Recorded Investment [Line Items]
Loans	1,100	2,063
Consumer Non-Performing | Individuals' loans for household and other personal expenditures
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 109	$ 60